Method of forming thermally stable photochromic images and product



Sept. 12, 1967 P. L. FORIS 3,341,330 METHOD OF FORMING THERMALLY STABLEPHOTOCHROMIG IMAGES AND PRODUCT Filed Jan. 16, 1964 4 Sheets-Sheet 1 IPHOTOCHROMIC MATERIAL LAYER i'l'ii PROVIDE PHOTOCHROMIC LAYERuLTRAvIoLET LIGHT F |G 2 LIGHT CONTROL STENCIL PHOTOCHROMIC LAYER QEXPOSE TO uLTRAvIoLET THERMALLY TIVE LIGHT IMAGE (DARK BLUE) SUPPORT IFIXING (HCI) VAPOR II ACIDIVAPOR COMPLEX OF coLoRLEss PHOTOCHROMIC LAYER(PALE YELLOW) ACID VAPOR COMPLEX OF COLORED THERMALLY STABILIZED IMAGE(DARK YELLOW I SUPPORT OLORLESS COMPLEX IN TIME AND MAY BE DEVELOPED BYULTRAVIOLET LIGHT TO GIVE GAS HOOD BLUE BACKGROUND 5 OPTIONAL REVERSINGINH4) VAPOR RESTORED PHOTOCHROMIC LAYER (COLORLESS AND SENSITIVE TOULTRAVIOLET LIGHT) ACID VAPOR SUBLIMES FORM ALKALI VAPORS (NH4) 0 RE ERsI MAGE) PETI E R E SO RIs T V BLUE a" 2osuPPoRT G HIS ATTORNYS Sept. 12,1967 P. L. FORIS 341,330

METHOD OF FORMING THERMALLY STABLE PHOTOGHROMIC IMAGES AND PRODUCT 4Sheets-Sheet 2 Filed Jan. 16, 1964 ALTERNATIVE METHOD STEPS F IG. 5 ACIDVAPORS(HCI) PROVIDE PHOTOCHROMIC PLATE STABILIZED PHOTO- CHROMIC LAYER(PALE YELLOW) ACID FIXING VAPORS(HCL) SUPPORT(GLASS) ULTRAVIOLET LIGHTTOCHR MIC COLORE ACID- I TREATED LAYER EXPOSE To ULTRAVIOLET LIGHTSTABILIZED IMAGE (OARK YELLOW) UPPORT 20 FIG T AALI VAP0RS(NH4) I{OPTIONAL GAS HOOD y REVERSING (NH4) VAPOR I EsTOREO PH TOcHROMIc LAYERPHOTOCHROMIC IMAGE(B| UE) swam? SUPPORT INVENTOR PETER L. FORIIS HISATTORNEYS Sept. 12, 1967 P. L. FORIS 3,341,330 METHOD OF FORMINGTHERMALLY STABLE PI'IOTOGHROMIC IMAGES AND PRODUCT Filed Jan. 16, 1964 4Sheets-Sheet 5 FIG, 3

S H m 0 M m S D N w E 5 TIME VARYING FROM (HCI VAPOR GOLORLESS ALKALINE(wa VAPOR EAT OR YELLOW TO lNFRA-RED LIGHT ULTRAVIO LET v H LIGHTALKALINE (NH VAPOR GEN VAPOR O L A H m C INVENTOR PETER L. FORIS Vguwv('3 c/ ms ATTORNEYS P. L. FORIS 3,341,330

HOTOCHROMIC Sept. 12, 1967 METHOD OF FORMING THERMALLY STABLE P IMAGESAND PRODUCT 4 Sheets-Sheet 4.

Filed Jan. 16; 1964 I F I G. 9

YELLOW mFR R D LIGHT T ULTRAVIOLET LIGHT s S E L R o L O C NON-HALOGEN0R ACID (s0 VAPOR ALKALINE (NH3) VAP NO REACTION TO NON-HALOGEN ACIDVAPOR I INVENTOR PETER L. FORIS ms ATTORNEYS United States Patent()fiice 3,341,339 Patented Sept. 12, 1967 of Maryland Filed Jan. 16,1964, Ser. No. 338,136 17 Claims. (Cl. 96-48) This invention relates toa method of creating thermally-stable photochromic images in layers andto means for converting such images into states having variouspersistences.

The images in contemplation are created in or on a layer of materialhaving sensitivity to ultraviolet light, the sensitivity of such layerbeing contributed by the presence of molecules of at least onephotochromic compound.

Photochromic compounds are those that undergo, what is believed to be,an internal molecular rearrangement, from a normal condition, whensubjected to ultraviolet radiation, and in which rearranged-conditioneach molecule radiation, and in which rearrangedcondition each moleculehas a light-absorption characteristic different than it had in thenormal condition. The molecules in the layer are affected by radiationindividually, and each molecule acts in an independent way to contributeto the overall visual concert-effect in a given area that is providedwith a sufficient profusion of such molecules. Of course, in any givenarea there must be enough of the photochromic material supplied so thatthe total effect on the human eye, or other photo-sensing means, is asthough there is a continuous layer of material to be sensed, the eye notbeing able to discern the individual contribution of any one of themolecules alone. Before this invention, of means for stabilizing imagesmade photochromically, such images could not be retarded from revertingto the normal state, because of the effect of ambient temperature,except by lowering the temperature of storage, and the images would belost. By this invention such images are not lost at room temperature orat higher or lower temperatures because of this reversion tendency.

The ultra-violet-light-rearranged-condition of the photochromic materialwill, for the purposes of this disclosure, be termed the colored stateand the normal condition will, in some instances, be termed thecolorless state, because most of the compounds evidence to the normalhuman eye, while in the colored state, in daylight or artificial light,a distinctive color within the visible spectrum and, on the other hand,in the colorless state evidence a colorless, a white, or a light-coloredhue. In an aggregation of such molecules which are used in associationwith each other in making the sensitized layer, some molecules may be inone state or another dependent on the light and heat energy to whichthey may have been subjected, that is to say-the colored state or thecolorless state, but all of the molecules, if the colored state, will inthe absence of ultraviolet light revert to the colorless or equilibriumstate in time, at a rate which increases with the ambient temperature.The reversion to the colorless state, therefore, causes a gradual fadingof a colored image with time. On the other hand, the colored state isassumed substantially instantaneously if a powerful source ofultraviolet light is applied to the photochromic compounds while themolecules are not bound in crystalline arrangement. This instantaneouschange, of course itself is effected molecule-by-molecule but in a shortenough time that the rate of change is not apprehended by the humanmind. The compounds differ among themselves as to the rate of reversionat a given temperature, the reversion to normal of the colored state ofthe molecules of some compounds being so fast at room temperature as torender them of no utility as a persistent color-contributor, whereas themolecules of other compounds remain in the colored state for hours,days, months, or years, depending on their molecular structure, in spiteof the ever-present urging of them to the normal colorless state byambient heat. In some compounds that should color in ultraviolet lightaccording to the behavior of analogous compounds, there is no evidencethat they do, but if a layer containing such compounds is fixed to acomplex chemical form according to this invention, before beingsubjected to ultraviolet light, such layer is useful for makingpermanent records with the tool of ultraviolet light. The differentcolors exhibited by the compounds of different kinds, together with thedifference in their rate of molecular reversion due to ambienttemperature, give a great opportunity for those who practice theinvention to select among the known compounds, of which there are many,in order to fit the condition of end use of the contemplatedphotochromic layer. The invention provides for the use of materials thatnever before could be used successfully for recording data. The behaviorof a layer will, to the eye, be a composite of the behavior of theindividual component molecules of whatever kind of photochromic materialis used, whether of the same kind of molecules or of different kinds ofmolecules interspersed together.

The condensation of Fischers Base with salicylaldehyde yields abenzo-indolinospiro pyran compound which in unsubstituted form is notuseful for record keeping with ultraviolet recording means, presumablybecause of ambient temperature conditions, but by the practice of thisinvention can be made useful for such purpose. The derivatives of suchcompounds which have proved very useful for recording of something lessthan permanently durable records now are more useful in that respectbecause of the formation of a thermally-stable complex of the compoundsaccording to this invention.

The fixing of a photochromic layer, as provided by this invention, haltsthe thermal reversion tendencies of the colored state of an imagetherein to the colorless state by the formation of a chemical complex ofsome kind, the exact nature of which is not fully understood but which,if the compound is a benzo-indolinospiro pyran, imparts a yellow colorto the layer, such color being pale in any portion not subjected toultraviolet light and of a deep yellow color where struck by ultravioletlight to form the image. However, this complex, while forming with themolecules in uncolored state does not render the uncolored moleculesfixed against being changed to the deep yellow colored complexed stateby subsequent ultraviolet light. Therefore, to make permanent records,the photochrornic layer may be treated with acid vapors before or afterbeing subjected to ultraviolet light.

A photochromic layer, if treated withthe acid vapor before or afterimage formation, may be reversed to the uncomplexed state with alkalinevapors. In this respect the invention provides far superior recordingsystems to that possible with any photochromic layers heretofore known.The photochromic art before this invention provided a rather evanescentrecording medium, not having the permanence required for durablerecords.

The photochromic layer of this invention may be an amorphous solid layerof the molecules laid down on a plate, sheet, or other support web, byevaporation of a concentrated liquid solution of the photochromiccompound, preferably applied thinly thereto, or it may be a solidsolution film of photochromic molecules in a polymeric film-formermaterial. It is within the scope of the invention to lay down on thesame support Web two or more different layers of material, distinguishedor not by their photochromic compound content or differing by actingtogether.

or in whole, within a with the vapors used for 'the state of thephotochromic material be it amorphous in solid state or in solution. Itis also within the concept of the invention to have different areas ofthe same record material support sheet sensitized with differentphotochromic compounds to give a distinctive color to the areasaccording to the compound used, thus one area may have data recorded inone distinctive color associated with the photochromic moleculesemployed therein, which is in contrast to that of an adjoining areawhich employs other types of photochromic molecules. Or the differentareas maybe distinguished by a diiference in the thermal persistencecharacteristics of different photochromic compounds used.

The recording of data may be made onto the novel record material by abeam or a cone of ultraviolet light of proper intensity and wave length,selectively applied directionally or by controlled intensity to thesheet. It will be apparent that light-control means such as shutters andstencils may be used togive selective operation of recording light, orother manner of light control may be used-such as filters, variations ofperiod of the lightapplication, or the use of two or more The fixing ofa photochromic layer with an acid vapor can be carried out by placingthe record material, in part gas-confining compartment, such as achemicalhood, and the same apparatus may be used the reversion of thelayer or images as will be described. The layer also may be selectivelyfixed or reverted by applying the vapors locally to an area desired tobe modified to the exclusion of the remainder of the layer, in anypattern or mode desired, such selection choice extending to a sheet ofsupport material coated with a layer on either side whereof one side istreated differentially to the other to obtain a combination of'imagesvisually observable in thickness.

The objects of the invention may be stated as being to provide means formaking thermally-stabilized photochromic layers, or images in suchlayers, in the form of chemical complexes; to provide means forreversing such fixed layers or images to their former uncomplexed state;and in general to provide a means for making permanent records withphotochromic materials that because of temperature conditions could notbefore be used.

The best known photochromic compounds now generally in use in makingrecord material for room temperature environmental use are thebefore-mentioned benzo-indo1inospiropyrans, having a normal structure inwhich substitutions may be made as follows:

to give room-temperature-operable photochromic materials even though notthermally-fixed. These materials and thousands of others are disclosedin United States Patent No. 3,100,778, which issued Aug. 13, 1963 on theapplication of Elliot Berrnan. The naphtho-pyran analog sources of lightand the l-phenyl derivative also may be mentioned, as Well as those inwhich the 3-carbon atom has been replaced with O or S.

The molecules of these compounds must have molecular freedom to changeconfiguration, and in such free state are easily converted to thecolored form by ultraviolet light while in solution, be it a liquidsolution or a solid solution, or in an amorphous solid state adherent toa support surface or included within a polymeric film support. Incrystalline form these compounds are difficultly converted to thecolored" state and difficultly revert to the colorless state so as torender them useless for record-making on a commercial scale. Moreover, aliquid solution of photochromic material which has had the photochromicmolecules converted to the colored state and allowed to dry into .acrystalline state, while the photochromic molecules are still in thecolored state, is fixed in such a converted state for all practicalshortterm purposes of record keeping, except for fading, but cannot bereversed successfully to active condition. With the use of the'best ofsuch compounds as heretofore known, images can be formed and exist inlayers made 7 thereof for some hours or days, without thermallyreverting to the colorless form, but such is not permanent enough forrecords that must be preserved. The number ofpossible photochromiccompounds available for use, is so great that a comprehensive listing ofthem would be impossible, but reference has been made to the UnitedStates Letters Patent which has disclosed photochromic compounds of thebenzo-indolinospiro pyran class, and to analogous compounds, that areoperative in connection with acid-vapor fixing and alkaline-vaporreversion, as claimed.

With the foregoing newly found principles in mind, the invention will bedescribed with reference to the drawings in which:

FIG. 1 shows a support 20 with the layer 21 comprising the photochromiccompound made either in an amorphous thin layer form or in a solutionstate (solid-solution form or liquid-solution form) or made as a thinsolid film of polymer material with the photochromic material insubstantially .an amorphous form distributed therein. From a conveniencepoint of view, and to prevent rapid sublimation of the material, asolution of the photochromic material in a thin polymer filmsolutionformer is to be preferred;

FIG. 2 shows the application of ultraviolet light to the layer 21 underthe control of an aperture-type of stencil which is to be taken astypical of light-image control means, the image of the letter N in darkblue colored form being represented in solid black on layer 21;

FIG. 3 shows the plate 20 bearing the image 21 being subjected to theacid-vapors of HCl within an enclosure, whereby the image N is fixed asa dark yellow complex with a pale yellow background shaded horizontally;

FIG. 4 shows the layer with the fixed photochromic image during itstreatment with ammonia vapor to reverse it to unfixed condition;

FIGS. 5, 6 and 7 refer to an alternative method, especially useful forphotochromic compounds not ordinarily operable at room temperature,wherein as shown in FIG. 5, the prepared photochromic layer is treatedfirst with acid vapor, before any image is impressed thereon, to fix thewhole layer to a pale yellow complex of the colorless form of thephotochromic material, to prepare it for the reception of images byultraviolet light; FIG. 6 shows the fixed colored 5 after beingsubjected to ultraviolet light; and FIG. 7

shows the photochromic colore image reversed by havrecord condition ofFIG.

photochromic layer image, according to the teachings of the inventionusing non-halogen acid vapors.

It Will be understood that the data to be recorded may be in terms ofthe colored portion of the layer or, on the other hand, in terms of thecolorless portion of the layer, the image being that portion affected byultraviolet light.

Inasmuch as the acid-complex of the part of a layer not subjected toultraviolet light will lose its acid vapor in a short time, bysublimation, the deep yellow of the image is retained and it may bevisually enhanced by flooding all of the layer with ultraviolet light,to color the background to give an enhanced visibility to the deepyellow of the image by color-contrast.

To form an amorphous layer of photochromic material by deposition from aliquid solution, the material is dissolved in a readily evaporableliquid applied in a thin film to a clean substrate surface, such as thesurface of a sheet of glass. Purity of these compounds tends towardsunwanted crystallization of such layers, if compounds are used singly. Amixture of closely related photochromic compounds will prevent thecrystallizing tendency through steric hindrance, An impure photochromiccompound is more apt to yield an amorphous film than a pure one.Substituents may be added to the compounds in various ways to achievethe best results, as more fully set forth in an application for UnitedStates Letters Patent, filed of even date by this applicant, wherein theforming of etching-fluid-resist images in photochromic compounds andamorphous films was disclosed and claimed.

Images, then, formed by the ultraviolet light in amorphous layers ofphotochromic material tend to fade and disappear in time through thermaldecay phenomena through heat energy obtained from the environment. Theinvention is directed to means for fixing such images against suchfading and disappearance, and to provide means for re-establishing thematerial by a reversing step so as to be responsive to furtherrecording, or to be more available for an optimal step of coloring thebackground to improve observation.

(A) PREPARATION OF A SOLID Amorphous layer on er support sheet Glass isa most suitable substrate for preparing films or layers according to theinvention, being smooth, rigid, transparent in its ordinary form, andhighly resistant to the materials employed. However, any equivalentsupport of metal, fiber, mineral, ceramic, polymer, and the like, may beused, as desired.

'First, a percent, by weight, solution of a typicalbenzo-indolinospiropyran which may exist in amorphous or solution formeither in the colorless or colored form at room temperature is preparedwith benzene, a highly useful one being the 6NO -8OCH derivative havingthe full formula l,3,3-trimethylindolino-6'-nitro-8-methoxybenzopyrylospiran. Second, an amount of the solution is appliedto a 4" x 4" glass plate of inch in thickness, which is spun at 500 rpm.at room temperature to remove the excess solution and to dry out thephotochromic material to a film of amorphous characteristics, whichcondition will persist for several days after which crystallization maystart.

If longer persistence in the amorphous state is required, half of thephotochromic material is replaced by the 7- phenyl-6'NO -8OCHderivative.

These films are dark blue in the colored substantially without hue inthe colorless subject to direct contact reaction with alkaline vapors.

(B) PREPARATION OF A SOLID Solution layer on a support sheet form, areform and are the acid and percent, by weight, solution of polyphenylphenol-form- (C) PREPARATION OF A COMPOSITE Layer system on: a supportsheet In this preparation two or more films or layers are formed on aninert plate in any arrangement, such as in superposition 0n the sameside or on different sides of the plate, or in overlapping relation onthe same side.

For instance, an initial layer of the 5Cl-6NO derivative of the parentbenzo-indolinopyrospiran may be laid down on a plate, an image createdthereon by ultraviolet light, and a complex formed with HCl vapor, afterwhich the background pale yellow complex is: removed with toluene,leaving the toluene-resistant image in dark yellow. Over this image isplaced a second amorphous layer of the 6NO -8OCH derivative, frombenzene solution, and a selected image is created with ultraviolet lightover the first image. The treatment of the plate again with HCl vaporresults in the two superimposed, but not necessarily alined images indeep yellow. Treatment of the whole plate with ammonia vapor yields apurple image on the bottom and a blue image over-laying it, as thecolored states of the two images are of dififerent hue.

Example I.-In this example, the preferred one, a sheet is preparedaccording to preparation instructions (A) and subjected to a cone ofultraviolet radiation, from a source producing radiation strong at 3000A., through a stencil as in FIG. 2 to produce the letter N in dark blueon a colorless background. The ultraviolet-light-exposed sheet then isplaced under a hood with HCl vapor, at atmospheric-pressure androom-temperature, for a few seconds, by which time the layer has turnedyellow with the image N appearing as a darker yellow hue. On removalfrom the HCl vapor the paler yellow portions of the layer will, within afew minutes, resume the colorless state due to sublimation of the vapor,leaving the N image in thermally-stabilized form of deep yellow hue. Thebackground will still be sensitive to ultraviolet light.

Example H.-The steps of Example I are followed with the further step offlooding the finished plate with ultraviolet light to produce ablue-colored background for the deep yellow N.

Example IlI.-A sheet prepared according to the instructions (B) is firstsubjected to HBr vapor for a few seconds at atmospheric pressure androom temperature to turn the film to the pale yellow complex. Next, thecomplexed layer is subjected to a selectively directed beam ofultraviolet light to trace an image which appears in dark yellow againsta pale yellow background.

Example I V.Example III is followed with the added steps of:

(1) subjecting the layer to alkaline vapor (NH to reverse the complex,

(2) making a new trace of different pattern with an ultraviolet lightbeam, and

(3) re-fixing the layer by again subjecting the layer to the HBr vapor,

thus :fixing as images both traces even though made at different times.

Example V.Any of the foregoing Examples I to IV is reversed to normal asto part of its area only by subjecting only such part of it to NH,vapor, the other parts being physically masked.

Example VI.-In this example photochromic layers are laid down on eitherface of a sheet made of material opaque to ultraviolet light but whichis transparent to visible light of the longer wave lengths. Images aremade with ultraviolet light on each layer of different subject matterand one or both sides are fixed by halogen acid vapors. Then at will,whatever area is to appear in normal colored state is treated with thereversing alkaline vapor. The sheet is viewed from one side or the otherby incident light or through the sheet by transmitted light to obtain acomposite effect.

Example Vll.-In this example a plate is prepared according toinstructions (A), an image is formed thereon by projecting ultravioletlight to it in the desired pattern. The plate then is treated with Svapor which combines with the colored image to form a stable yellowcomplex but which leaves the background unconverted in the originalcolorless form, as S0 though an acid vapor is not an acid of the halogengroup. This background is still sensitive to ultraviolet light for morerecording and fixing.

The stable SO -yellow images may be reversed with alkaline vapors as wasthe case with the other examples.

In this example of the invention, a recording made on the colorlessstate of the layer may be fixed only after the recording, whereas if thefixing" is with a halogen type of acid vapor it may be carried outbefore the application of ultraviolet light.

It will now be understood and appreciated that the steps of theinvention may be of combinations using the fixing and reversing stepswhen needed, and that combinations of photochromic compounds and meansfor forming images to complement or supplement one another, areprovided.

The invention is characterized also by the distinguishing use of halogenacid vapors and non-halogen acid vapors, as the occasion of use demands.

The invention therefore is claimed in a manner to embrace variouscombinations of steps all coming within the inventive concept.

What is claimed is:

1. The method of forming thermally-stable images made on platessensitized with a photochromic spiropyran material, including the stepsof forming the desired image on the plates by application thereto ofultraviolet light in selected image representing areas; and thereaftersubjecting the plate having the so-formed image to hydrohalic acidvapors.

2. The method of forming thermally-stable images made on platessensitized with photochromic spiro-py-ran material, including the stepsof forming the desired image on the plates with ultraviolet light afterhaving subjected the sensitized plates to hydrohalic acid vapors.

3. The method of forming thermally-stable photochromic images, includingthe steps of:

(a) providing a layer of record material sensitized with a spiro-pyranphotochromic compound;

(b) subjecting the layer to hydrohalic acid vapor to form a chemicalcomplex of the acid vapor and the compound; and

(c) creating an image in the layer by directing ultraviolet lightthereto in image-representing pattern.

4. The method of claim 3 followed by the step of subjecting the layer tosubliming environmental conditions to drive the vapor from thebackground areas of the layer not subjected to the ultraviolet lightthus leaving the background colorless. V

5. The method of thermally-stabilizing ultraviolet-lightformedthermally-decayable images formed in layers sensitized with spiro-pyranphotochromic material, con- I sisting of the step of subjecting thelayers to acid vapors selected from the group consisting of hydrohalicacid vapors and sulfur dioxide vapors.

6. A method of recording consisting of the steps of:

(a) forming a layer of photochromic material of the spiro-pyran type;

(b) recording data on the layer by directing ultraviolet employed in agreat variety light thereto to make thermally-decayable images;

and

(c) stabilizing the images by subjecting them to acid vapors selectedfrom the group consisting of hydrohalic acid vapors and sulfur dioxidevapors long enough to form acid complexes with the photochromicmolecules.

7. A method of recording consisting of the steps of:

(a) forming an image on a layer of photochromic material of thespiro-pyran type by exposure of it to ultraviolet light in theimage-representing areas, and keeping such image from thermal decay bycooling said layer;

(b) and later subjecting the record to acid vapors selected from thegroup consisting of hydrohalic acid vapors and sulfur dioxide vaporsbefore subjecting the layer to higher temperatures apt to cause thermaldecay of the image, said image being reversible to its state in which itis subject to thermal decay by application of ammonia vapors.

8. A method of recording consisting of the steps of:

(a) preparing a record by successively recording data on a layer ofphotochromic material of the spiropyran type at diflerent time periods,and

(b) then fixing the layer by subjecting the layer to acid vaporsselected from the group consisting of hydrohalic acid vapors and sulfurdioxide vapors.

9. A method of recording, including the steps of preparing aphotochemically-sensitive film comprising compounds ofbenzo-indolinospiropyran structure; forming an image thereon by applyingultraviolet light in selected patterns; and flooding the film with anacid vapor selected from the group consisting of hydrohalic acid vaporsand sulfur dioxide vapors until a yellow complex forms involving theacid and the photochromic molecules.

10. The method of claim. 9 followed by the step of treating the complexwith ammonia vapor to reverse the fixing of the image.

11. A method of recording data including the steps of forming an imagein a layer sensitized with photochromic material by means of firstforming a complex of the photochromic material of the layer withhydrohalic acid vapors; and forming the image of the data in terms of anacid complex of the photochromic material of the layer with ultravioletlight.

12. A method of recording consisting of the steps of:

(a) forming an amorphous layer of spiro-pyran photochromic material on asupport plate;

(b) recording data on the layer by ultraviolet light to makethermally-decayable images; and

(c) stabilizing the images against thermal decay by subjecting them toacid vapors selected from the group consisting of hydrohalic acid vaporsand sulfur dioxide vapors long enough to form complexes with thephotochromic material.

13. The product of the process of claim 12.

14. The method of forming thermally-stable images by ultraviolet lightincluding the steps of:

(a) preparing a film including amorphous photochromic material of thespiro-pyran type;

(b) forming a photochromic colored-state image on (a) with ultravioletlight; and

(c) fixing the image against thermal decay of by exposure to hydrohalicacid vapors.

15. The product of the process of claim 14.

16. The use of the method of claim 14 torform a thermally-fixedphotochromic image followed by the step of treating the image byexposure of the film to ammonia vapor to reverse the image to theunfixed colored state.

17. The method of forming thermally-stable images by use of ultravioletlight, including the steps of:

(a) preparing a film of a solution of spiro-pyran photochromic materialin a polymeric film-forming material;

(b) forming an image on the film with ultraviolet light;

and

the film (c) fixing the image against thermal decay by applying to thefilm acid vapors selected from the group consisting of hydrohalic acidvapors and sulfur dioxide vapors.

References Cited UNITED STATES PATENTS 1 0 OTHER REFERENCES Theoreticaland Experimental Investigation of Photo- Chromic Memory Techniques andDevices, ASD Technical Report, 61-70, December 1961, AD #273,512, pp. 1,8, 336, and 356.

NORMAN G. TORCHIN, Primary Examiner. C. E. DAVIS, Examiner.

7. A METHOD OF RECORDING CONSISTING OF THE STEPS OF: (A) FORMING ANIMAGE ON A LAYER OF PHOTOCHROMIC MATERIAL OF THE SPIRO-PYRAN TYPE BYEXPOSURE OF IT TO ULTRAVIOLET LIGHT IN THE IMAGE-REPRESENTING AREAS, ANDKEEPING SUCH IMAGE FROM THERMAL DECAY BY COOLING SAID LAYER; (B) ANDLATER SUBJECTING THE RECORD TO ACID VAPORS SELECTED FROM THE GROUPCONSISTING OF HYDROHALIC ACID VAPORS AND SULFUR DIOXIDE VAPORS BEFORESUBJECTING THE LAYER TO HIGHER TEMPERATURES APT TO CAUSE THERMAL DECAYOF THE IMAGE, SAID IMAGE BEING REVERSIBLE TO ITS STATE IN WHICH IT ISSUBJECT TO THERMAL DECAY BY APPLICATION OF AMMONIA VAPORS.